I have experienced this myself...

Now sometimes when I am reading about new things..my mind wanders...and I allow it..it opens up new ideas...and ways at looking at the topic....which I do not think is bad...

everyone learns differently...so what worked for me...might not neccessarlly work for you.

When I am having this problem....and my mind wanders WAY off topic.....

I take notes...helps me focus on the topic...and force my mind to comprehend and pay attention.....also allows me to add my ideas to the topic...some reference for my mind to remember the original idea...

Not sure why it happens in the first place...maybe something is bothering you????

MLF
Edit

I totally agree with rapier57

Healthy body..healthy mind

I work out when I need ti clear my head....or have something that bothers me...get the blood flowing to the brain...